Recently, water-absorbing materials have been widely used in various fields including baby diapers, adult diapers, hygiene products such as sanitary products, and agricultural/gardening supplies. For example, Japanese Laid-Open Patent Application No. 2004-155806 describes an absorbent substrate in which cellulose ether is cross-linked via polyglycidyl ether.
Further, Japanese Laid-Open Patent Application No. 2005-179253 (“JP '253”) describes a gel sheet containing a low-substituted cellulose ether having a molar substitution of 0.05 to 1.0, aqueous cellulose ether having a molar substitution of 1.1 to 1.4, and water. JP '253 describes that this sheet is a gel sheet having adequate strength and elasticity while containing much water, and having further improved feel and adherence to the skin.
Further, Japanese Laid-Open Patent Application No. 2005-194200 (“JP '200”) describes cosmetics containing a self-cross-linked alkyl cellulose derivative hydrogel as a substrate, wherein the cosmetics have excellent stability, safety, biodegradability, as well as excellent sense of use, appearance, etc. In JP '200, alkyl cellulose is cross-linked by radiation to form a hydrogel.
On the other hand, also known are compositions containing a water-absorbing polymer (such as hydroxypropyl cellulose) and a water-absorbing silica gel.
For example, Organic-Inorganic Hybrid Material, Technical Information Institute Co. Ltd, 2000, pp. 329-334 describes a hydrogel using a hybrid of polyvinyl alcohol and SiO2. This document describes that the degree of swelling (evaluated according to the length change of a piece of the specimens) and the tensile stress of the resultant hydrogel change according to the amount of tetraethoxysilane (amount of SiO2) . Water absorbency of this hybrid, however, is not as high as that of hydroxypropyl cellulose. This hybrid also does not have liquid absorbency for ethanol.
Furthermore, U.S. Pat. No. 6,275,728 describes use of films containing a water-absorbing polymer as a substrate for iontophoresis which allows transdermal absorption of a drug by electric current. Although a film containing hydroxypropyl cellulose and silica gel is described as an example in the patent, the molecular weight of the hydroxypropyl cellulose is unclear and a solid silica gel is added. Accordingly, an additional test was carried out using a hydroxypropyl cellulose having the same molecular weight as those used in the Examples of the present invention. However, a substrate having sufficient properties as a liquid-absorbing substrate was not obtained.
Furthermore, Japanese Laid-Open Patent Application No. 7-309972 (“JP '972”) describes a biodegradable composition containing hydroxypropyl cellulose and tetraethoxysilane or its hydrolytic polycondensates. The composition described in JP '972 is intended to be utilized for, for example, surface material of capsules for agrichemicals, fertilizer or the like and is not intended to be used as a water-absorbing substrate. In the document, the molecular weight of the hydroxypropyl cellulose is unclear. According to the Examples, tetraethoxysilane heated and hydrolyzed in the presence of water is used. Therefore, an additional test was carried out using hydroxypropyl cellulose having the same molecular weight as those used in the Examples of the present invention. However, a substrate having sufficient properties as a liquid-absorbing substrate was not obtained.
Furthermore, POLYMER, Volume 35, No. 25, 1994, pp. 5565-5570 describes production of a complex of hydroxypropyl cellulose and silica by adding water and hydrochloric acid to hydroxypropyl cellulose having a weight-average molecular weight of 60 thousand and tetraethoxysilane and hydrolyzing and polycondensing the tetraethoxysilane. A substrate having sufficient properties as a liquid-absorbing substrate was not obtained from an additional test of this method.